Corn is Ready to Topdress

Dr. Heather Darby, UVM Extension

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The corn crop is off to a good start this season. Many fields are at the stage of rapid nutrient uptake. A key decision that needs to be made by farmers is whether additional nitrogen should be applied to meet the crop’s need for the remainder of the season. The PSNT is one of the best tools available to gauge if supplemental nitrogen is needed. This test offers a diagnostic method for evaluating the nitrogen supplying capacity of the soil just prior to the plants period of greatest N demand. Corn plants begin their greatest uptake of N around the 5th leaf stage, or when the plants are approximately 12 inches tall at the center of the whorl. In contrast to traditional soil tests, the soil sample is collected from the top 12 inches of soil when corn plants are approximately 12 inches tall or at least one week before planned sidedressing. Growers who apply animal or green manures to their fields are especially likely to benefit from the PSNT. The organic matter in manure contributes nitrogen that is not well accounted for by other soil test methods. For assistance in collecting samples for PSNT testing contact your local extension specialist. Although the PSNT is one of the best tools we have to measure available nitrate it is not without short falls. The reliability of results from this testing procedure is heavily dependent on making sure the samples are collected, handled, and processed correctly. Even if sampling, handling, and processing are done correctly, the reliability of this test when values are low is sometimes questionable. If the values are high–greater than 25 ppm–then the odds are good that no additional N will be needed for the crop.

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